WPF and Silverlight Chart: multiple series databinding

Recently I needed a chart with multiple series (legend items) which would be able to display data from an OLAP database, like this:

The standard Chart requires to define each legend item in XAML, which doesn’t satisfy me, because I don’t know the actual number of returned items from a data source. So I decided to extend this control. Fortunately, there was a little work: only 3 properties and 1 method.